Stabilization of chromium nickel stainless steels



United States Patent 3,203,789 STABILIZATION 9F CHRQMKUM NICKEL STAINLESS STEELE:

William A. Morgan, Qttawa, Ontario, Robert .1. McClure, Beiis Qorners, Ontario, and David E. C. King, (ittawa, Ontario, Canada, assignors, by direct and mesne assignments, to Her Majesty The Queen in right of Canada as represented by the Minister of Mines and Technical Surveys N Drawing. Filed Sept. 27, 1961, Ser. No. 141,005

2 Claims. (Cl. 75-130.5)

This invention relates to stabilization of chromium nickel stainless steels.

The problem of sensitization in the production of chromium nickel stainless steels is well recognized. Sensitization is the precipitation of chromium carbides at the austenitic grain boundaries at temperatures of 450-900" C., leaving adjacent areas depleted in chromium and thus lowering the corrosion resistance of the depleted areas.

To inhibit this problem it has been common practice to employ one of the two following procedures:

(a) the carbon content is lowered to such an extent (03% maximum) that the small amount of chromium carbide formed is not enough to render the steelsensitized. This procedure necessitates a longer melt down period and consequently higher cost.

(b) a carbide forming element, usually comprising titanium or columbium, that forms a more stable carbide than chromium carbide, is added. Both titanium and columbium are relatively costly.

It is an object of this invention to provide a method of stabilizing chromium nickel stainless steels by utilizing a novel stable carbide forming element.

Another object is to providea chromium nickel stainless steel having therein a stable uranium carbide.

In accordance with the invention, uranium, which has an afiinity for carbon greater than chromium, is added to a chromium nickel stainless steel to produce dispersed uranium carbides and thus avoid the formation of chromiurn carbides at the grain boundaries.

The uranium is added in an amount corresponding approximately to its stoichiornetric proportion to carbon. Such proportion is 20 times the percentage of carbon by weight in the case of uranium monocarbide. In accordance with the invention, therefore, uranium is added in the proportion of 16 to 20 times the carbon present.

If sulphur is present in the steel, it is necessary to add suflicient additional uranium to combine with such sulphur. As pointed out in copending application filed concurrently herewith and entitled Free Machining Steels and Method of Making Same, the so added uranium improves the properties of resulphurized free machining steels. In such copending application, uranium is added to a resulphurized stainless steel in a percentage 1 to 9 times that of the sulphur present. Thus, in a resulphura ized chromium nickel stainless steel, uranium is added in sufiicient quantity to combine both with the carbon and sulphur present. The resultant steel is, therefore, not only stabilized but it has the improved properties resulting from the substitution of the small widely dispersed particles of uranium sulphides for the long stringers of manganese sulphides.

The stabilization of this type of steel has been evaluated by using the following procedure:

(1) Cut a sample from a rolled bar.

(2) Solution heat treatment (1 hour at 1900 F. followed by a water quench).

(3) Heat treat at 650 C. (1200 F.) for 1 hour followed by a water quench. This step is to assure the precipitation of chromium carbide if the steel is not stabilized.

(4) Weigh and determine the surface area of the sample.

(5) Place in boiling HNO (70%) for 24 hour periods.

(6) Determine the weight loss per unit area per 24 hour period. This weight loss is the measure of the stabilization. The smaller the weight loss the more stable is the steel.

Types 321 and 347 stainless steels have been used as the standard sinc both are common stabilized grades.

The following are the results of tests:

Weight loss per unit area (mg/sq. cm.) Stabilizer, 24 hour periods Type of Steel Percent 1st 2nd 3rd 4th 5th Average Nonc 1. 43 1. 43 .49% Ti .83 .63 .79 .95 l. .80 .70% Cb .915 .77 .72 .94 .85 None 1. 62 1.17 2. 2G 1. 49 l. 51 1. 61 .Q% U 1. 25 .54 1.15 .52 .54 .80

It is thus evident that uranium has been fully as eifective as a stabilizing element as titanium and eolumbium. Moreover, its cost, especially if depleted uranium is employed, is currently less and thus use of uranium is more economical.

Furthermore, the use of uranium possesses other advantages. Because of its afiinity for other elements such as oxygen, nitrogen, and sulphur, it acts to clean the steel of sulphides and other impurities. In addition, it improves the high temperature properties of the steel, since uranium carbide is more stable at high temperatures than either columbium or titanium carbide. It may be mentioned in this connection that while columbium-stabilized steel is substantially more expensive than titanium-stabilized steel, the latter adversely affects the high temperature properties since titanium carbide is not fully stable at high temperatures.

It will be apparent that other carbide formers, such as titanium and columbium, could be employed in combination with uranium.

\Ve claim:

1. A method of stabilizing a chromium nickel stainless steel which comprises adding to a melt of said steel uranium in a percentage thereof corresponding approximate- 1y to its stoichiometric proportion of carbon present in said melt to form uranium carbides and avoidance of the formation of chromium carbides therein.

2. A method of improving the characteristics of a free machining chromium nickel stainless steel which comprises adding to a melt of said steel uranium in a percentage thereof corresponding to its stoichiometric proportion of carbon present in said melt plus its stoichiometric proportion of sulphur present in said melt to form therein uranium carbides and avoidance of the formation of chromium carbides and to form therein uranium sulphides and avoidance of the formation of manganese sulphides.

References Cited by the Examiner FOREIGN PATENTS 720,008 11/31 France.

OTHER REFERENCES Titanium in Iron and Steel, pp. 41, 53 and 54, edited by Comstock, pub. in 1955 by John Wiley and Sons, Inc., New York.

DAVID L. RECK, Primary Examiner.

RAY K. WINDHAM, Examiner. 

2. A METHOD OF IMPROVING THE CHARACTERISTICS OF A FREE MACHINING CHROMIUM NICKEL STAINLESS STEEL WHICH COMPRISES ADDING TO A MELT OF SAID STEEL URANIUM IN A PERCENTAGE THEREOF CORRESPONDING TO ITS STOICHIOMETRIC PROPORTION OF CARBON PRESENT IN SAID MELT PLUS ITS STOICHIOMETIC PROPORTION OF SULPHUR PRESENT IN SAID MELT TO FORM THEREIN URANIUM CARBIDES AND AVOIDANCE OF THE FORMATION OF CHROMIUM CARBIDES AND TO FORM THEREIN URANIUM SULPHIDES AND AVOIDANCE OF THE FORMATION OF MANGANESE SULPHIDES. 